


The Backdoor Boogie

by equilateral_asshat



Category: Gravity Falls
Genre: F/M, So many different ways to say buttsex, pinecest - Freeform, pinescest - Freeform
Language: English
Status: Completed
Published: 2018-12-16
Updated: 2018-12-16
Packaged: 2019-09-20 08:14:19
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,604
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/17019021
Author URL: https://archiveofourown.org/users/equilateral_asshat/pseuds/equilateral_asshat





	The Backdoor Boogie

The night had begun like any other night, really.

Dipper and Mabel each sat in their respective rooms, listening through cracked doors for familiar sounds; their mother’s gentle footsteps, and the heavier, more tired ones of their father shortly thereafter, followed by the sound of a bedroom door at the end of the hallway locking, the latch on their door noisy.

Dipper mentally began to count, staring at his computer screen but not really taking anything in, leg bouncing. It took his father close to five minutes from door-latching to flat out dead asleep, but his mother would probably be reading for a few more minutes yet, as it was one of the rare moments she had the peace and quiet to do so.

This stretch of time was akin to agony to Dipper Pines. Only when he was fairly certain that both parents were asleep, would he take the chance of crossing the five feet of space between his bedroom door, to that of Mabel’s. After all, the both of them were meant to be asleep at this hour, and merely being up past their proposed bedtime would be reason enough for them to get into trouble. If their parents found out why they were awake at this hour, there’d be hell to pay.

Of course, if their mother or father discovered the twins, limbs or lips (or both) entangled, the fact that it was past their bedtime would be the smallest thing they’d get into trouble for. They’d likely be grounded, or worse, separated.

Roughly twenty minutes had passed when Dipper poked his head out of his doorway and stared at his parent’s door. No light came from the crack under the door, so he felt confident that is was now safe. He stepped lightly on his toes, crossing the gap in two steps. He gently tapped his knuckles against Mabel’s door, and cracked it open when he heard her clear her throat.

“Hey,” he muttered as he slipped in, staring at his feet. Partially because looking directly at her felt somehow wrong, sinful, despite knowing what he would see. He’d seen it dozens of times before, when this taboo relationship began a year or so ago. Even if she was his girlfriend, trying to sneak a peek at Mabel in the buff seemed perverse.

“Look at me, you adorable doofus,” she teased. He felt his cheeks tinge crimson, but when he lifted his gaze, he partially deflated, shoulders slumping in relief as he then rolled his eyes. As opposed to sleeping in naught but what she was born with, tonight, Mabel was wearing an old, red, flannel shirt of his, long and baggy on her more narrow frame. From the waist down, all he could tell that she wore were a pair of mismatched socks, one solid purple, the other grey and pink with individual toes of alternating colors. He wasn’t sure she ever wore matching socks at all anymore.

“Sorry,” he mumbled, running fingers through his messy hair. She smirked and sat up a bit straighter.

“Don’t apologize for pretendin’ to be a gentleman, brother mine,” she reassured him. He shook his head.

“No, for taking so long. Mom’s been reading a new book and I wanted to make sure she was good and rightly asleep before I tried to sneak over,” he explained, moving to sit next to her on the bed. His butt hit the mattress a bit too hard, causing her to bounce for a moment, before she fell into leaning against him, head on his shoulder.

“Naw,” she said, taking his hand in one of her own, using a finger on her other hand to draw imaginary lines on the backs of his hand. “I understand. She’s a pretty hardcore book nerd. I mean, you didn’t get that from dad.”

It was his turn to smirk, his thumb rubbing over hers as he shifted to plant a gentle kiss against her temple. “What did I get from him then? My smooth moves maybe?”

“Ohmigawd gross, don’t you even suggest that again!” she laughed, swatting him playfully on the knee with her free hand. “And clearly you got your dorkiness from him.”

“How dare!” he scoffed, pulling his hand free to prop his fists on his hips in mock anger. “I developed this dorkiness all on my own thank you very much, and have subsequently used it to thoroughly seduce you!”

“Hmm, I suppose that’s a fair,” she interrupted her phrase with a kiss, her lips pressing to his with a gentle touch, “assessment.”

That first kiss was all it took before the twins were lip locked, Mabel swiveling around to plop herself in her brother’s lap with haste. She loved being on top for this, having the control of the situation. She knew Dipper could probably wrestle her down just as easily, but he never bothered. He was content to turn the reins over to her, it seemed, and they never openly discussed the matter.

The first half of their twice-a-week-after-dark meetings almost always went this way. Their lips pressing together time and again, her hips gyrating in his lap to drive their twin points of heat together. His hands clutching her hipbones, only to slide down the back of her pants, nails dragging softly against the smooth skin of her posterior on their way back up to grasp her hips again. They were both likely to be fired up to full desire in no time.

Roughly ten minutes of necking, groping, and dry-humping led to Mabel reaching back with one hand, sightlessly pulling open her bedside table’s drawer, as she had done countless times before. Into this drawer she reached, fingers tapping about in search of the familiar crinkle of plastic squares with little circular bulges.

Tap. Tap-tap. Nothing, nada, zilch. She straightened herself up, breaking off a kiss much to Dipper’s dismay.

“Wha-” he began, but she held up a finger.

“Just trying to find you a raincoat, bro-bro, but it seems…” After another five seconds of fruitless labors, she simply spun around to look in the drawer. Oh, oh no. No no no. “Aw heck!”

“What’s wrong, Mabes?” Dipper scooted to sit up, awkwardly bending his legs to hide the tent in his pajama pants. Of course, she knew it was there. She’d caused it, and she’s seen it plenty of times before, PJ’s or no, but he still had that strange, teenage shyness associated with boners.

“What’s wrong is I’m hot to trot, your lil soldier man is standing at full attention, I haven’t gotten laid in over a week and I am _out of condoms._ ”

“What, no, we bought that big box last time! We haven’t used…” he paused, then counted on his fingers, lips moving silently as Mabel balled her hands into fists in frustration. “We should still have at least one or two left!”

“Uuuuhh,” Mabel croaked, eyes darting to one side nervously. “I might have needed one to keep my phone dry last week when it was raining to beat all,” she noted. “And um… might have made a glitter bomb with the other one?”

“Mabel?!” he groaned, flopping back on the bed, one hand clasped over his eyes, the other dragging one of her pillows into his lap. “Why didn’t you buy more then?!”

“It’s your turn!” she hissed. Well, that was true, however…

“Fair, but you never told me we needed them!” He huffed, sitting up and burying his face in his hands. “And it’s not like I can just stroll out of here at ten to midnight, drive to the megamart down the road, and buy some willy nilly! If I get caught leaving the house on a school night mom and dad will kill me, let alone what I’m leaving to do!”

“Well, then,” she pouted, “what are our options?”

“Well, there’s the tried and true circle jerk,” he suggested, and she scrunched up her nose. “We could always finish one another off, hands, mouths, however you want?”

“Noooo, it’s not enough!” she grumped, grabbing the pillow off of his lap to flop it over her face as she slumped back on the bed. The rest of her reply, though muffled by the pillow, was still perfectly audible to Dipper. “I mean, I need to feel you _inside_ me, bro. I’m yearnin’ for that full feeling.”

“Well, we can’t risk what might happen if we were to do this without protection, Mabes,” he sighed, rubbing the top of her head. “Much as I might like to, mom won’t take you to get birth control yet, and I have no confidence in my pull-out game.”

“Ew, don’t say that phrase,” she teased, smacking him with the pillow now. “It just makes it sound so unsexy!”

There was a brief chuckle between them, followed by a near minute of full silence. Mabel’s mind was racing with options, while Dipper was resigning himself to an evening with unrestricted internet access in his bedroom for some quick relief, when Mabel perked up. There was a nervous cough from her throat, low and cautious to avoid being too loud.

“So, um, we technically could still do it, I guess,” she mumbled, fiddling with a length of her hair.

“Mabes, no, I mean I wouldn’t mind just feeling _you_ but I don’t want to risk-”

“There wouldn’t be any risk!” she insisted. “We just… have to improvise.” Her brother stared at her, eyebrow perked, hands raised, palms up as if to say ‘explain’. She stared hard back, but he failed to develop telepathic powers in that moment. She tinged pink.

“I could give you a backstage pass?” More staring. “You can putt from the rough?” Still, staring. “Ugh, don’t make me say it out loud!”

“Say what?” he chuckled, finding this amusing as it was bewildering.

“Yanno, go dumpster diving! Ride the backroads, bowl in the basement, poke the forbidden eye!”

“Mabel, use english!” he snorted. She got redder in the face.

“Plunder the booty, visit the rump rodeo, do the backdoor boogie! Use nature’s condom!” Her voice was rising in pitch, while staying at a whisper, as she got progressively darker and darker as her blush deepened.

Something in Dipper’s mind managed to translate somewhere between ‘rump rodeo’ and ‘backdoor boogie’, and he started to blush. She was seriously suggesting that she would… with him?

“Uh, Mabes, are you serious? You wanna have bu-” His inquiry was stamped out with a hand slapped to his mouth.

“Don’t say it, it doesn’t sound sexy, and if I hear it said in such vulgar terms I’m gonna be dead, d-e-d DED, Dipper Pines! And that would make you a necrophiliac, and that’s even worse!”

“First, Mabel,” he said, moving her hand from his mouth, “presuming I would make the nasty with your body if you shed your mortal coil insists that you have little to no faith in my standards, so shame on you.” She stuck her tongue out, as she had, after all, only been teasing. So even if he was just teasing back, it was still gross to think about. “Secondly, I’m just making sure you really, _really_ want to do, well, _that_.”

“Bro, I’m horned up enough right now, that I even suggested it. You never have, and despite how good you are with computers, I still know how to find your gross internet history so I know you have at least passing interest.” His turn to blush again, which emboldened her. “And, I do have a little bottle of lube, for my days where I’m feeling kinda dry, so it’s not like I’m not prepared.”

“I mean, I just, uh,” he stammered, hands wringing at the old shirt he wore for pajamas. Mabel’s hand appeared on his thigh, and then quickly cupped that familiar bulge between his legs.

“Well, I’m not hearing you say no, and it seems like part of you is really into the idea of exploring uncharted mystery tunnels,” she teased, giving him a playful rub through the fabric. He merely gulped, then nodded.

“I… okay, yes. Yeah, I’m down. I mean I’ll do it if you want to, I just d-” Once again he was silenced, this time by lips instead of a hand. The long, drawn out kiss put his nervous worry to rest. “So, we’re really gonna…?”

“Yep,” she chimed, hand revisiting the drawer of her bedside table to dig around for a moment, before producing a small bottle. It had a pink cap, one that you sort of pushed down on one side to open the nozzle. She handed it over to him, then took a deep breath as she wiggled to sit on the edge of the bed. With a quick, practiced motion, a pair of frilly pink panties, with purple polka-dots on it, slid over her legs, and was then launched across the room towards her hamper like one would fire a rubber band. She missed by a few feet, but that mattered very little.

“So, um, what positio-oh.” His question was answered before his mouth could finish spewing it out. His sister was now on all fours on her mattress, rump turned to face him.  His tongue darted out across his lips as his nerves rattled. He had certainly dreamed about this sort of thing, had pondered it, had fantasized a time or two when he had to spend a night alone, Mabel-less, in his own room. But it was happening, now. He was really going to get to do this.

Mabel initially stiffened as she felt his hands land on her backside. They spread her cheeks apart, and this was followed by a very hushed, enthusiastic ‘ohfuck’ on her brother’s part. A nervous tremble ran through her frame; she was truly, completely down for this, but she was also scared. Would she enjoy it? Would it hurt? Was he too big for… this? One of the hands left her flesh, and then she felt two fingers press into her cleft, the tips coated in a cold, slippery substance. Her back arched, butt clenching reflexively as she squeaked.

“Ack, sorry, j-just, cold,” she murmured into a pillow. Dipper nodded wordlessly, the fingers rubbing in small circles. As the lubricant warmed, the gentle action of the massage helped her relax. It felt good, though somewhat alien, to have somebody touching her like that. She flexed the pucker of flesh against his fingertips; half to tease, and half to give herself an idea of just how tight she might be.

“Are you ready?” His words were gentle, soft, and spoken methodically. He was trying to hide his nervous excitement, and had she not known all of her brother’s tells, he might have succeeded. She gave a small, curt nod, then cleared her throat before letting out a long sigh.

“Y-yeah, just…” she paused, biting her lower lip. “Please be slow.” She felt his lips brush against the small of her back, which gave her goosebumps in the best way.

“Always,” he breathed against her, the tip of one finger giving a testing prod. Each of the twins, in this moment, were more than a little shocked at just how readily the first inch or so of his finger sank. Mabel clenched in surprise, and Dipper froze. “You good?”

She nodded again, falling to her elbows, burying her face in her pillow. The sensation was different. Not bad, but just, strange. She could tell there was definitely a finger in her booty, and she could tell that it didn’t bother her.  She gave her hips a small roll backwards, to let him know it was okay to continue.

Dipper was mentally comparing how this felt versus all the times he’d had his fingers in what his sister lovingly (a bit too lovingly in his opinion) enjoyed referring to as _the Madame Priscilla_. The major difference was that when she squeezed, she squeezed hard. Apparently the backdoor was a much stronger ring of muscle than he had accounted for. Also, while still soft and sticky in that way that insides are sticky, it was different. Not gross (she was clean as a whistle, from what he could tell), but the musculature that held things in place created different contours. This particular orifice was also not self-lubing, which forced a sort of quiet, analytical concentration on his part to ensure he didn’t do anything to hurt her, as the finger worked in and out, as well as wiggling side to side.

“M… maybe a little more lube,” Mabel mumbled. “A-and… another finger?” Dipper hummed in acknowledgement, his finger slipping away. She heard the snap of the lube bottle opening, then closing again, and once again that telltale cold of fresh lube on flesh. She shuddered, but didn’t clench up this time. Dipper was very slow and patient, merely maintaining pressure with his digits as she worked her hips back, a gasp escaping her when she felt herself stretch out around his fingers. “D-dang…”

“Too much?” Dipper asked, his hand moving back. Mabel’s own shot out and latched around his wrist, her head shaking. “Mabel, I don’t want to hurt you.”

“Doesn’t hurt,” she squeaked. “Just… intense.” Maybe she lied a little, it kind of pinched? It wasn’t anything she couldn’t handle, at the least, but there was a slight tinge of discomfort mixed in with the oddly pleasurable feeling of being penetrated. Another thing that was nagging at Mabel’s mind, was that this was a very different shift in the power dynamic the bedroom usually held.

Normally she was in charge! She set the pace, she was on top. While she knew Dipper would never hurt her, and was going to almost be _too_ careful for awhile, she wasn’t in direct control. She was basically surrendering herself to him and something about this caused her innards to flutter with excitement. She didn’t want to lose that spark they had always had when she held the reins, but a small, slowly growing part of her was indeed getting off on the idea that she was giving up so much control of the situation.

None of this conundrum had struck Dipper. He, instead, was mesmerised, watching his sister’s flesh grip his fingers, shifting as his wrist twisted them this way and that, how she flexed when he moved them a little faster than she expected. Even his baggy PJ’s were starting to feel too tight for his liking. He kissed against the small of her back again, humming as he did. His lips vibrated on her skin, and he felt her posture slacken somewhat.

“Ready for the main event?” he asked, his voice hushed but excited. He couldn’t hide it now.

Mabel nodded wordlessly, panting into a drool soaked pillow. She almost whined at the sensation of sudden emptiness when his fingers vanished, then had to fight the urge to tense up when she felt his weight shift, followed by what was clearly a lube-slick wang pressing against her pucker. Her legs wobbled, her hips trembled.

“S-slow, please,” she reiterated, booty wiggling almost imperceptibly. His hands rested on her waist, and soon she felt pressure against her. It was slow, and seemed to have stubborn intent behind it, Dipper holding his position until, with a grunt, and then a sigh, Mabel relaxed and pushed back.

The twins sucked in a breath in unison, each hissing through clenched teeth. Mabel was fighting the urge to tighten up, and Dipper was doing his best to hold still while she adjusted. What struck him the most was not how tight she felt, as he had anticipated that, but instead was how _hot_ she felt inside. Somehow, though only separated through a thin membrane, the backdoor seemed to be several degrees warmer than the front.

Mabel, meanwhile, was contending with how raw and primal this new sensation was. Normal sex was almost a relief, being penetrated was like having an itch scratched and sighing at the cool sensation when _Madame Priscilla_ was taking the pounding. This, this was demanding. She already felt full, but knew she could feel fuller. She knew she could increase this sensation several times over if she could just get more of him in there. Whatever discomfort she felt only seemed to add violent passion to the act, and she craved more.

Gentle nudges from one set of hips, and rolls backwards from the other, and after a few minutes the twins’ thighs were reunited. Dipper bent low, hooking his chin on her shoulder while he hugged around her midsection, his body shaking with adrenaline just as much as hers. His breath was hot and muggy against her ear, and the squeaking moans that quietly escaped the back of his throat were more encouraging than she had thought any noise from him could be.

“Gawds, Dip,” she groaned into the pillow, which was becoming more drool-slick by the second. “You f-feel even bigger back there.”

“W-well, you’re definitely t-tighter,” he growled. His hips rolled against her, grinding him about her insides, and she shuddered. She might have even let out a muffled swear or two, which was very unlike Mabel. “You okay for me to start movin’ more?”

“ _Please_ ,” she hissed, her rear pressing to him. “I need you to do it, I wanna feel you fill me up again and again.”

Dipper happily, silently obliged, saying nothing as his thighs moved away from hers, and she whimpered as his retreating member left an empty sensation in its wake, only to gasp happily as he refilled her, thighs meeting her own again. Once he was all the way in, she pressed back again, trying to beg for more without talking.

They moved together like this for a few minutes, Mabel finding that the implicit trust of this act was turning her on just as much as the act itself. She trusted Dipper to not hurt her, to only make her feel good, and by Aoshima’s many fists was he ever making her feel GOOD. She started to meet him halfway on his thrusts, a desperation for more building up in her midsection. A fire she never knew was there grew in her, and she knew that only more of this could snuff that flame out for now.

Blindly, she groped under herself until she found Dipper’s hands. Taking one by the wrist, she shifted it between her legs with a whimper.

“Touch me,” she quietly begged. “Oh my frick Dip, pleeeeaaase touch me, I’m already so close ohmyfriggingaaawwd.”

“A-anything for you,” he sighed in her ear, his lips catching the rim of it as two dextrous, well trained fingers found her bud. She squeaked and wiggled with increased fervor, a chuckle rising in his throat. His fingers avoided touching the most sensitive spots, those he knew how to find in the dark, blindfolded and likely with his ears plugged. He could navigate this part of his favorite person in the world by touch alone, and he knew how to drive her wild with it, to tease her until she was ready to burst. Given how heavy she was already breathing from just the rutting alone, he figured he could coax a moan or two from her with ease.

He wasn’t wrong.

As her brother’s fingertips danced circles around her most sensitive of places, she squirmed this way and that to try and make him slip up, to get him to touch some part of her that would set off the hair trigger she knew needed squeezing. She mentally cursed how well he knew her ladybits, how he could always touch where he knew it would drive her mad without getting her off until the last second. And, worst of all, was she had explicitly instructed him on it the first few times they had ventured into no-pants-dance territory. The student had become the master…

Dipper’s reasons for stalling were twofold. First, he knew Mabel loved it. The way her hips shivered whenever he came close to the spots that would send her flying over the edge betrayed any irritated sounds she tried to make. Secondly, he wanted to time this perfectly, and he was rapidly approaching the inevitable apex of his own pleasure. It wasn’t often they managed to climax together, and he wanted this time-maybe the only time, as far as he knew, that he would be buried to the hilt in Mabel’s backside-to end with a simultaneous explosion of bliss.

“Diiippeeerrrrrr,” Mabel cried quietly into her pillow. Her fingers were trying to grasp his wrist, to try and steer him into getting her off. She didn’t know how much more of this she could take; the raw, almost feral nature of having her butt stuffed with the teasing, torturous fingers that danced circles delicately around her clit and folds, occasionally brushing just enough to keep her on edge, was about to switch her self control off. It was everything she could do not to scream, in fact.

“A-almost… there…” he grunted into her ear, his hips meeting hers a few more times. That tightness, that building pressure in his midsection was full to bursting. His legs shook with tremors, fighting to keep himself from pushing forward too hard. His fingers swirled around her nethers, drawing closer, closer. And then, in the next instant, each twin hit their limit.

Mere moments before he felt his length throb that telltale first throb, Dipper mercifully rubbed his fingers over Mabel’s clit, and her orgasm was immediate and powerful. Every muscle in her body tightened in sync, her body pulsing with orgasm. Her squeal was drowned out in her pillow. She felt herself tighten around Dipper’s Dip-stick, her back passage clenching over and over again.

That clenching was what sent Dipper past the brink, his body clinging tightly to his sister’s as he came, wave after wave of of his essence being milked by her involuntary squeezes. At least, at first they were involuntary. Each time she tensed around him, he would hiss, his own body responding in kind by tensing up, causing him to throb and dribble a bit more. His vision went white as he screwed his eyes shut tightly, willing himself to stay perfectly still. Mabel would have none of that, and every few seconds gave another squeeze, chuckling into her bedding when he hissed inwardly.

Eventually, his body did what most male bodies did after sex, and between his slowly softening flesh and his sister’s attempts to milk him dry, he was suddenly evicted from her body. They both gasped, and without a word they rolled to the side. Dipper spooned behind Mabel, his face buried in her neck.

“So, uh,” he mumbled after minutes of total silence, save for labored breathing between them. “Was that good, or bad, or…?”

Mabel sniggered to herself for a moment, slowly rolling over. “Well,” she mused, her gaze meeting his, “it was different. Good different. Intense? Raw? I wanted to feel full and by-golly, badonk-a-donk bonkin’ delivers.”

“So, it’s definitely in the ‘repeat performance’ category?” he asked. The brat actually had the gall to let hopeful optimism leak into his inquiry, too! She’d probably bite him for that later,as she was often wont to do when he got cocky with her.

“I mean, I think we can probably extend how fast we go through condoms from now on.”

They both burst into a fit of giggles at that, Dipper trying his damnedest to not laugh as he kissed her forehead.

Quietly, each twin added “ _The Backdoor Boogie_ ” to their lists of favorite positions.

-END-


End file.
